EP4611256A1

EP4611256A1 - Photovoltaic power generation system and insulation detection method

Info

Publication number: EP4611256A1
Application number: EP24202469.3A
Authority: EP
Inventors: Fanyu HU; Nianan PAN; Jinhu CAO
Original assignee: Sungrow Power Supply Co Ltd
Current assignee: Sungrow Power Supply Co Ltd
Priority date: 2024-02-28
Filing date: 2024-09-25
Publication date: 2025-09-03
Also published as: CN118074627A

Abstract

A photovoltaic power generation system and an insulation detection method are provided. The system includes a switch, a DC-AC circuit, a controller and at least one DC-DC circuit. An input end of each of the at least one DC-DC circuit is configured to connect n photovoltaic strings, n is an integer greater than or equal to two, and an output end of the at least one DC-DC circuit is connected to an input end of the DC-AC circuit. Positive electrodes of all the photovoltaic strings are grounded through the switch. The controller is configured to close the switch and determine a photovoltaic string where an insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings, if the insulation fault occurs at a negative electrode of the photovoltaic power generation system to ground. If an insulation fault occurs at the negative electrode of the photovoltaic power generation system, the positive electrodes of all the photovoltaic strings are controlled to be grounded, and a short circuit occurs between a negative electrode of a photovoltaic string where the insulation fault occurs and the ground. As a result, a current path is formed between the negative electrode of the photovoltaic string and the ground, and currents of other photovoltaic strings flow into the photovoltaic string where the insulation fault occurs, in a direction opposite to a direction of a current of the photovoltaic string where the insulation fault occurs, so that the photovoltaic string where the insulation fault occurs can be determined.

Description

FIELD

The present disclosure relates to the technical field of insulation fault detection, and in particular to a photovoltaic power generation system and an insulation detection method.

BACKGROUND

A photovoltaic power generation system includes photovoltaic strings, a DC-DC circuit and an inverter circuit. Generally, an input end of the inverter circuit is connected to multiple DC-DC circuits, and an input end of each of the multiple DC-DC circuits is connected to multiple photovoltaic strings connected in parallel.
During operation, an insulation fault may occur in a photovoltaic string, causing safety hazards to the operation of the photovoltaic power generation system. Therefore, insulation fault detection is usually performed on the photovoltaic power generation system.
In the conventional technology, whether an insulation fault occurs in the photovoltaic power generation system is determined by detecting an insulation impedance of the photovoltaic power generation system to ground. And, if the insulation fault occurs in the photovoltaic power generation system, a photovoltaic string where the insulation fault occurs fails to be determined, which is not conducive to on-site troubleshooting of the insulation fault.

SUMMARY

In view of this, a photovoltaic power generation system and an insulation detection method are provided according to the present disclosure, to accurately determine a photovoltaic string where an insulation fault occurs.
A photovoltaic power generation system is provided according to an embodiment of the present disclosure. The photovoltaic power generation system includes a switch, a DC-AC circuit, a controller and at least one DC-DC circuit. An input end of each of the at least one DC-DC circuit is configured to connect n photovoltaic strings, where n is an integer greater than or equal to two; an output end of the at least one DC-DC circuit is connected to an input end of the DC-AC circuit; positive electrodes of all the photovoltaic strings are grounded through the switch; and the controller is configured to close the switch and determine a photovoltaic string where an insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings, if the insulation fault occurs at a negative electrode of the photovoltaic power generation system to ground.
In an embodiment, the photovoltaic power generation system further includes at least n-1 current detection devices. Negative electrodes of the n photovoltaic strings are connected to each other. Each of the at least n-1 current detection devices is connected between a negative electrode of a corresponding photovoltaic string and a negative junction corresponding to the photovoltaic string, and the current detection device is configured to detect a current of a branch where the photovoltaic string is located.
In an embodiment, the controller is configured to determine, if a current of an i-th photovoltaic string is detected as negative, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, where the i-th photovoltaic string is one of the n photovoltaic strings.
In an embodiment, the photovoltaic power generation system further includes n-1 current detection devices. Negative electrodes of the n photovoltaic strings are connected to each other to form n-1 negative junctions. n-2 current detection devices of the n-1 current detection devices each are connected between two adjacent negative junctions of the n-1 negative junctions, and the remaining one current detection device of the n-1 current detection devices is connected between a negative electrode of a first photovoltaic string and a negative junction corresponding to a second photovoltaic string.
In an embodiment, the controller is configured to determine, if a current of an i-th photovoltaic string is detected as negative and an absolute value of the current is maximum, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, where the i-th photovoltaic string is one of the n photovoltaic strings.
In an embodiment, a negative input terminal and a negative output terminal of the at least one DC-DC circuit are connected to each other.
In an embodiment, a negative input terminal and a negative output terminal of the at least one DC-DC circuit are disconnected from each other.
In an embodiment, the photovoltaic power generation system further includes a diode. For each of the at least one DC-DC circuit, positive electrodes of the n photovoltaic strings connected to the input end of the DC-DC circuit are connected to an anode of the diode corresponding to the DC-DC circuit, a cathode of the diode is connected to a first terminal of the switch, and a second terminal of the switch is grounded.
In an embodiment, the photovoltaic power generation system further includes a current limiting device. The number of the current limiting device and the number of the at least one DC-DC circuit are equal to each other. For each of the at least one DC-DC circuit, the current limiting device corresponding to the DC-DC circuit, the diode corresponding to the DC-DC circuit and the switch are connected in series to form a branch, and the branch is connected between a positive input terminal of the DC-DC circuit and the ground.
In an embodiment, the photovoltaic power generation system further includes a current limiting device. The number of the current limiting device and the number of the switch each are one. For each of the at least one DC-DC circuit, the current limiting device, the diode corresponding to the DC-DC circuit and the switch are connected in series to form a branch, and the branch is connected between a positive input terminal of the DC-DC circuit and the ground.
An insulation detection method for a photovoltaic power generation system is further provided according to an embodiment of the present disclosure. The photovoltaic power generation system includes a switch, a DC-AC circuit and at least one DC-DC circuit. An input end of each of the at least one DC-DC circuit is configured to connect n photovoltaic strings, n is an integer greater than or equal to two, and an output end of the at least one DC-DC circuit is connected to an input end of the DC-AC circuit; and positive electrodes of all the photovoltaic strings are grounded through the switch. The insulation detection method includes: closing the switch and determining a photovoltaic string where the insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings, if the insulation fault occurs at the negative electrode of the photovoltaic power generation system to the ground.
In an embodiment, the photovoltaic power generation system further includes at least n-1 current detection devices. Negative electrodes of the n photovoltaic strings are connected to each other, each of the at least n-1 current detection devices is connected between a negative electrode of a corresponding photovoltaic string and a negative junction corresponding to the photovoltaic string, and the current detection device is configured to detect a current of a branch where the photovoltaic string is located. The determining a photovoltaic string where the insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings includes: determining, if a current of an i-th photovoltaic string is detected as negative, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, where the i-th photovoltaic string is one of the n photovoltaic strings.
In an embodiment, the photovoltaic power generation system further includes n-1 current detection devices. Negative electrodes of the n photovoltaic strings are connected to each other to form n-1 negative junctions; n-2 current detection devices of the n-1 current detection devices each are connected between two adjacent negative junctions of the n-1 negative junctions, and the remaining one current detection device of the n-1 current detection devices is connected between a negative electrode of a first photovoltaic string and a negative junction corresponding to a second photovoltaic string. The determining a photovoltaic string where the insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings includes: determining, if a current of an i-th photovoltaic string is detected as negative and an absolute value of the current is maximum, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground.
It can be seen that the present disclosure has the following beneficial effects.
In the photovoltaic power generation system according to the embodiments of the present disclosure, if an insulation fault occurs at the negative electrode of the photovoltaic power generation system, the positive electrodes of all the photovoltaic strings are controlled to be grounded, and a short circuit occurs between a negative electrode of a photovoltaic string where the insulation fault occurs and the ground. As a result, a current path is formed between the negative electrode of the photovoltaic string and the ground, and currents of other photovoltaic strings flow into the photovoltaic string where the insulation fault occurs, in a direction opposite to a direction of a current of the photovoltaic string where the insulation fault occurs. Therefore, the photovoltaic string where the insulation fault occurs can be determined by detecting a magnitude and a direction of the current of each of the photovoltaic strings according to the embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic diagram of a photovoltaic power generation system according to an embodiment of the present disclosure;
Figure 2 is a schematic diagram of a photovoltaic power generation system according to another embodiment of the present disclosure;
Figure 3 is a schematic diagram of a photovoltaic power generation system according to another embodiment of the present disclosure;
Figure 4 is a schematic diagram illustrating DC-DC circuits in a photovoltaic power generation system in common negative connection according to an embodiment of the present disclosure;
Figure 5 is a schematic diagram illustrating DC-DC circuits in a photovoltaic power generation system not in common negative connection according to an embodiment of the present disclosure;
Figure 6 is a schematic diagram of a photovoltaic power generation system in common negative connection according to another embodiment of the present disclosure;
Figure 7 is a schematic diagram of a photovoltaic power generation system not in common negative connection according to another embodiment of the present disclosure;
Figure 8 is a schematic diagram illustrating a photovoltaic power generation system including a current limiting resistor according to an embodiment of the present disclosure;
Figure 9 is a schematic diagram illustrating a photovoltaic power generation system including current limiting resistors according to another embodiment of the present disclosure; and
Figure 10 is a flowchart of an insulation detection method for a photovoltaic power generation system according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In order to make the above objective, features and advantages of the present disclosure more apparent and understandable, hereinafter, the embodiments of the present disclosure are described in further detail in combination with the drawings and specific implementations.
Reference is made to Figure 1, which is a schematic diagram of a photovoltaic power generation system according to an embodiment of the present disclosure.
In the embodiment of the present disclosure, the photovoltaic power generation system includes a DC-AC circuit 100 and at least one DC-DC circuit.
The number of the DC-DC circuit is not specifically limited in the embodiment of the present disclosure. That is, an input end of the DC-AC circuit is connected to one DC-DC circuit or multiple DC-DC circuits. In addition, the number of the DC-AC circuit 100 provided in the photovoltaic power generation system is not specifically limited in the embodiment of the present disclosure. The photovoltaic power generation system includes one DC-AC circuit or multiple DC-AC circuits. To facilitate description, the following embodiments are described by taking one DC-AC circuit as an example.
As shown in Figure 1, an input end of the DC-AC circuit 100 is connected to M DC-DC circuits as an example.
For example, an input end of each of the DC-DC circuits is configured to connect n photovoltaic strings, where n is an integer greater than or equal to two, and output ends of the M DC-DC circuits are connected to the input end of the DC-AC circuit 100. As shown in Figure 1, a first DC-DC circuit is described as an example. String1+ and String 1- represent a positive electrode and a negative electrode of a first photovoltaic string (a first photovoltaic string from a positive electrode towards a negative electrode of the first DC-DC circuit is taken as the first photovoltaic string) respectively, String2+ and String2- represent a positive electrode and a negative electrode of a second photovoltaic string respectively, String3+ and String3- represent a positive electrode and a negative electrode of a third photovoltaic string respectively, ..., and String n+ and String n- represent a positive electrode and a negative electrode of an n-th photovoltaic string respectively. Similar to the first DC-DC circuit, each of a second DC-DC circuit, ..., to an M-th DC-DC circuit is connected to corresponding photovoltaic strings, which is not described in detail herein.
In the embodiment of the present disclosure, the number of photovoltaic strings connected to the input end of each of the DC-DC circuits is identical. In other embodiments, the number of photovoltaic strings connected to an input end of a DC-DC circuit is different from the number of photovoltaic strings connected to the input end of another DC-DC circuit.
The input end of the DC-DC circuit is connected to multiple photovoltaic strings. As a result, if an insulation fault occurs in a photovoltaic string to the ground, the photovoltaic string fails to be determined, i.e., the photovoltaic string where the insulation fault to the ground occurs fails to be accurately determined.
A solution for determining a photovoltaic string where the insulation fault to the ground occurs is provided according to the embodiment of the present disclosure as follows. If it is detected that an insulation fault occurs at a negative electrode of the photovoltaic power generation system to the ground, the photovoltaic power generation system is controlled to stop operating, positive electrodes of all the photovoltaic strings are controlled to be grounded, and the photovoltaic string where the insulation fault to the ground occurs is directly determined based on a magnitude and a direction of a current of each of the photovoltaic strings.
The solution is described in detail in conjunction with the drawings hereinafter.
Reference is made to Figure 2, which is a schematic diagram of a photovoltaic power generation system according to another embodiment of the present disclosure.
The photovoltaic power generation system according to the embodiment of the present disclosure includes a switch K, a DC-AC circuit 100, a controller and at least one DC-DC circuit. The number of the at least one DC-DC circuit is not specifically limited in the embodiment of the present disclosure. That is, the input end of the DC-AC circuit 100 is connected to one DC-DC circuit or multiple DC-DC circuits.
An input end of each of the at least one DC-DC circuit is configured to connect n photovoltaic strings, where n is an integer greater than or equal to two. An output end of the at least one DC-DC circuit is connected to the input end of the DC-AC circuit 100.
Positive electrodes of all photovoltaic strings are grounded through the switch K. Each DC-DC circuit is individually described as follows. The input end of each of the DC-DC circuits is connected to corresponding n photovoltaic strings. A positive input terminal of a first DC-DC circuit is connected to positive electrodes of corresponding n photovoltaic strings, and a negative input terminal of the first DC-DC circuit is connected to negative electrodes of the n photovoltaic strings. The positive electrodes of the n photovoltaic strings are grounded through the switch K, that is, the positive input terminal of the first DC-DC circuit is grounded through the switch K. Similarly, a positive input terminal of a second DC-DC circuit is connected to positive electrodes of corresponding n photovoltaic strings, and a negative input terminal of the second DC-DC circuit is connected to negative electrodes of the n photovoltaic strings. The positive electrodes of the n photovoltaic strings are grounded through the switch K, that is, the positive input terminal of the second DC-DC circuit is grounded through the switch K. Similarly, a positive input terminal of an M-th DC-DC circuit is connected to positive electrodes of corresponding n photovoltaic strings, and a negative input terminal of the M-th DC-DC circuit is connected to negative electrodes of the n photovoltaic strings. The positive electrodes of the n photovoltaic strings are grounded through the switch K, that is, the positive input terminal of the M-th DC-DC circuit is grounded through the switch K.
The controller (not shown in Figure 2) is configured to close the switch K and determine a photovoltaic string where an insulation fault occurs based on a magnitude and a direction of a current of each of the photovoltaic strings, if the insulation fault occurs at a negative electrode of the photovoltaic power generation system to the ground.
The type of the switch K is not specifically limited in the embodiment of the present disclosure, and the controller may control an on-off state of the switch K.
It should be understood that, if an insulation fault occurs in the photovoltaic power generation system to ground, it is determined whether the insulation fault occurs based on an insulation impedance of the photovoltaic power generation system to the ground. For example, if the insulation impedance of the photovoltaic power generation system to the ground is less than a preset impedance, it indicates that the insulation fault occurs. According to the embodiment of the present disclosure, if the insulation fault occurs at the negative electrode of the photovoltaic power generation system, the insulation fault is further accurately located.
If the insulation fault occurs at the negative electrode of the photovoltaic power generation system, the positive electrodes of all the photovoltaic strings are controlled to be grounded, and a short circuit occurs between a negative electrode of a photovoltaic string where the insulation fault occurs and the ground. As a result, a current path is formed between the negative electrode of the photovoltaic string and the ground, and currents of other photovoltaic strings flow into the photovoltaic string where the insulation fault occurs, in a direction opposite to a direction of a current of the photovoltaic string where the insulation fault occurs. Therefore, the photovoltaic string where the insulation fault occurs can be determined by detecting the magnitude and the direction of the current of each of the photovoltaic strings according to the embodiments of the present disclosure.
In the embodiment of the present disclosure, the photovoltaic power generation system is further provided with a diode, in order to avoid mutual interference between different DC-DC circuits resulted from that positive electrodes of photovoltaic strings connected to different DC-DC circuits are connected to each other if the switch is closed. In this way, if the switch is closed, positive electrodes of photovoltaic strings connected to a DC-DC circuit are not directly connected to positive electrodes of photovoltaic strings connected to another DC-DC circuit. For each of the DC-DC circuits, positive electrodes of the n photovoltaic strings connected to the input end of the DC-DC circuit are connected to an anode of the diode corresponding to the DC-DC circuit, a cathode of the diode is connected to a first terminal of the switch, and a second terminal of the switch is grounded. That is, the positive input terminal of each of the DC-DC circuits is connected to the switch through the corresponding diode. As shown in Figure 3, for example, M DC-DC circuits correspond to M diodes. A positive input terminal of a first DC-DC circuit is grounded through a first diode D1 and the switch K connected in series in sequence. A positive input terminal of a second DC-DC circuit is grounded through a first diode D2 and the switch K connected in series in sequence. A positive input terminal of an M-th DC-DC circuit is grounded through a first diode DM and the switch K connected in series in sequence.
Multiple DC-DC circuits in two connection manners are described hereinafter in conjunction with the drawings, that is, negative input terminals of multiple DC-DC circuits are connected to each other, and negative input terminals of multiple DC-DC circuits are disconnected from each other.
In an embodiment, negative input terminals of multiple DC-DC circuits are connected to each other. Since output ends of all the DC-DC circuits are connected to the input end of the DC-AC circuit, positive output terminals of the multiple DC-DC circuits are connected to each other, and negative output terminals of the multiple DC-DC circuits are connected to each other. In a case that negative input terminals of the multiple DC-DC circuits are connected to each other, the negative input terminals and the negative output terminals of the multiple DC-DC circuits have a same potential and are connected to each other.
In the photovoltaic power generation system according to the embodiments of the present disclosure, the solution for locating an insulation fault varies with a location of the current detection device. Implementations in which the current detection devices are provided at two locations are described hereinafter in conjunction with the drawings. A current detected by the current detection device varies with the location of the current detection device. If a current detection device is provided at a branch where the photovoltaic string is located, the current detection device detects a current of the single photovoltaic string. If a current detection device is provided at a junction branch, the current detection device detects a current that is a sum of currents of multiple photovoltaic strings.
Reference is made to Figure 4, which is a schematic diagram illustrating DC-DC circuits in a photovoltaic power generation system in common negative connection according to an embodiment of the present disclosure.
As shown in Figure 4, each of the current detection devices is provided at a branch where a corresponding photovoltaic string is located, and is configured to detect a current of the single photovoltaic string.
As shown in Figure 4, negative input terminals and negative output terminals of M DC-DC circuits are connected to each other, i.e., the M DC-DC circuits are in common negative connection.
The photovoltaic power generation system according to the embodiment of the present disclosure further includes at least n-1 current detection devices. For n photovoltaic strings, at least n-1 current detection devices are provided to determine the photovoltaic string where the insulation fault occurs. For example, at least four current detection devices are provided to locate an insulation fault of five photovoltaic strings. It should be understood that, five current detection devices may be provided for five photovoltaic strings.
The negative electrodes of the n photovoltaic strings are connected to each other. That is, the n photovoltaic strings are connected in parallel.
Each of the current detection devices is connected between a negative electrode of a corresponding photovoltaic string and a negative junction corresponding to the photovoltaic string, and the current detection device is configured to detect a current of a branch where the photovoltaic string is located, that is, the current detection device is configured to detect a current of a negative electrode of a single photovoltaic string.
The controller is configured to determine, if a current of an i-th photovoltaic string is detected as negative, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, where the i-th photovoltaic string is one of the n photovoltaic strings.
For the photovoltaic power generation system in common negative connection, in which the negative input terminals and the negative output terminals of all the DC-DC circuits (where the DC-DC circuits each are a common two-level Boost circuit or a three-level flying capacitor Boost circuit) are connected to each other, the negative input terminals and the negative output terminals have a same potential. In this case, a current path is formed between each of photovoltaic strings connected to all the DC-DC circuits and the ground, and the current detection device for the photovoltaic string acquires a current of a corresponding branch, so that the insulation fault can be located based on a magnitude and a direction of the current, as shown in Figure 4.
If an insulation fault occurs at a negative electrode of the first photovoltaic string to the ground, an equivalent impedance Riso of the negative electrode to the ground is less than a safety threshold of the insulation impedance, the switch K is closed, and a current path is formed between each photovoltaic string and the ground. As shown in Figure 4, dash dot lines illustrate current paths of normal photovoltaic strings (the second photovoltaic string to the n-th photovoltaic string) where no insulation fault occurs, a dashed line illustrates a current path of the photovoltaic string (the first photovoltaic string) where the insulation fault occurs, and a dotted line illustrates a common current path. It can be seen from the current paths that, a current detected by a current detection device for the first photovoltaic string is equal to a sum of currents detected by current detection devices for other normal photovoltaic strings, and is in a direction opposite to a direction of the currents of other normal photovoltaic strings, so that it is determined that the insulation fault occurs at the negative electrode of the first photovoltaic string.
Similarly, if an insulation fault occurs at a negative electrode of the n-th photovoltaic string, a current detected by the current detection device for the n-th photovoltaic string is in a direction opposite to a direction of currents detected by current detection devices of other normal photovoltaic strings, and the current of the n-th photovoltaic string is a sum of the currents of the other normal photovoltaic strings.

For example, two DC-DC circuits are provided, and an input end of each of the two DC-DC circuits is connected to five photovoltaic strings. If an insulation fault to the ground occurs at a negative electrode of a first photovoltaic string, a second photovoltaic string, a third photovoltaic string, a fourth photovoltaic string, or a fifth photovoltaic string connected to one of a first DC-DC circuit and a second DC-DC circuit, currents detected by the current detection devices for various photovoltaic strings are shown in Table 1. Wmn represents a current detection device for an n-th photovoltaic string connected to an m-th DC-DC circuit.

Table 1. Currents of photovoltaic strings if a short circuit occurs between a negative electrode of a photovoltaic string and the ground

	W11	W12	W13	W14	W15	W21	W22	W23	W24	W25
First photovoltaic string connected to first DC-DC	-4.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A
Second photovoltaic string connected to first DC-DC	0.5A	-4.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A
Third photovoltaic string connected to first DC-DC	0.5A	0.5A	-4.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A
Fourth photovoltaic string connected to first DC-DC	0.5A	0.5A	0.5A	-4.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A
Fifth photovoltaic string connected to first DC-DC	0.5A	0.5A	0.5A	0.5A	-4.5A	0.5A	0.5A	0.5A	0.5A	0.5A
First photovoltaic string connected to second DC-DC	0.5A	0.5A	0.5A	0.5A	0.5A	-4.5A	0.5A	0.5A	0.5A	0.5A
Second photovoltaic string connected to second DC-DC	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	-4.5A	0.5A	0.5A	0.5A
Third photovoltaic string connected to second DC-DC	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	-4.5A	0.5A	0.5A
Fourth photovoltaic string connected to second DC-DC	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	-4.5A	0.5A
Fifth photovoltaic string connected to second DC-DC	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	0.5A	-4.5A

From currents detected by various current detection devices in Table 1, it can be seen that a current flowing through a branch of a negative electrode of a photovoltaic string where the insulation fault to the ground occurs is equal to a sum of currents of other normal photovoltaic strings, and is in a direction opposite to a direction of the currents of other normal photovoltaic strings, so that it is determined that the insulation fault occurs at the negative electrode of the photovoltaic string.
It can be seen from the first row of Table 1 that, the insulation fault to the ground occurs at the negative electrode of the first photovoltaic string connected to the first DC-DC circuit. The current detected by a current detection device W11 is -4.5A, and currents corresponding to other photovoltaic strings each are 0.5 A. In the photovoltaic power generation system in common negative connection, the insulation fault can be located just by a direction of the detected current of each of the photovoltaic strings. The current detected by the current detection device W11 is negative, and currents detected by other current detection devices are positive. Furthermore, considering the magnitudes, an absolute value of the current detected by the current detection device W11 is a sum of absolute values of currents of all other photovoltaic strings in common negative connection, which can be obtained from Table 1. Detection errors of the above values are ignored.
Hereinafter, the DC-DC circuits in the photovoltaic power generation system not in common negative connection are described.
Reference is made to Figure 5, which is a schematic diagram illustrating DC-DC circuits in a photovoltaic power generation system not in common negative connection according to an embodiment of the present disclosure.
The photovoltaic power generation system shown in Figure 4 is different from the photovoltaic power generation system shown in Figure 5 in that for each of DC-DC circuits in Figure 5, a negative input terminal and a negative output terminal of the DC-DC circuit are disconnected from each other, and a diode is connected between the negative input terminal and the negative output terminal. As shown in Figure 5, although negative electrodes of photovoltaic strings connected to each of the DC-DC circuits are connected to each other, negative input terminals of different DC-DC circuits are disconnected from each other, i.e., the photovoltaic power generation system is not in common negative connection.
In the photovoltaic power generation system not in common negative connection, the negative input terminal and the negative output terminal of each of the DC-DC circuits are disconnected from each other. For example, the DC-DC circuit is a common symmetrical three-level Boost circuit, and negative electrodes of photovoltaic strings connected to different DC-DC circuits are different in potential. In this case, no current path is formed between photovoltaic strings connected to normal DC-DC circuits where no insulation fault occurs and the ground, as shown in Figure 5.
If an insulation fault occurs at a negative electrode of the first photovoltaic string to the ground, an equivalent impedance Riso of the negative electrode to the ground is less than a safety threshold of the insulation impedance, the switch K is closed, and a current path is formed between photovoltaic strings and the ground. As shown in Figure 5, dash dot lines illustrate current paths of normal photovoltaic strings (the second photovoltaic string to the n-th photovoltaic string) where no insulation fault occurs, a dashed line illustrates a current path of the photovoltaic string (the first photovoltaic string) where the insulation fault occurs, and a dotted line illustrates a common current path. It can be seen from the current paths that, a current detected by a current detection device for the first photovoltaic string is equal to a sum of currents detected by current detection devices for other normal photovoltaic strings, and is in a direction opposite to a direction of the currents of other normal photovoltaic strings, so that it is determined that the insulation fault occurs at the negative electrode of the first photovoltaic string. In addition, no current path is formed between photovoltaic strings connected to DC-DC circuits where no insulation fault occurs and the ground, and the current is basically zero.
Similarly, if an insulation fault occurs at a negative electrode of the n-th photovoltaic string, a current detected by the current detection device for the n-th photovoltaic string is in a direction opposite to a direction of currents detected by current detection devices of other normal photovoltaic strings, and the current of the n-th photovoltaic string is a sum of the currents of the other normal photovoltaic strings.
For example, two DC-DC circuits are provided, and each of the two DC-DC circuits is connected to five photovoltaic strings. If an insulation fault to the ground occurs at a negative electrode of a first photovoltaic string, a second photovoltaic string, a third photovoltaic string, a fourth photovoltaic string, or a fifth photovoltaic string connected to one of a first DC-DC circuit and a second DC-DC circuit, currents detected by the current detection devices for various photovoltaic strings are shown in Table 2.

From Table 2, it can be seen that if the current detection device W11 for the first photovoltaic string is not provided, and each of currents detected by other current detection devices is greater than zero, it is determined that the insulation fault is located at the negative electrode of the first photovoltaic string. Therefore, at least four current detection devices are provided to locate an insulation fault of five photovoltaic strings. Correspondingly, if one DC-DC circuit is connected to n photovoltaic strings, at least n-1 current detection devices are provided.

Table 2. Currents of photovoltaic strings if a short circuit occurs between a negative electrode of a photovoltaic string and the ground in a photovoltaic power generation system not in common negative connection

	W11	W12	W13	W14	W15	W21	W22	W23	W24	W25
First photovoltaic string connected to first DC-DC	-2.0A	0.5A	0.5A	0.5A	0.5A	0A	0A	0A	0A	0A
Second photovoltaic string connected to first DC-DC	0.5A	-2.0A	0.5A	0.5A	0.5A	0A	0A	0A	0A	0A
Third photovoltaic string connected to first DC-DC	0.5A	0.5A	-2.0A	0.5A	0.5A	0A	0A	0A	0A	0A
Fourth photovoltaic string connected to first DC-DC	0.5A	0.5A	0.5A	-2.0A	0.5A	0A	0A	0A	0A	0A
Fifth photovoltaic string connected to first DC-DC	0.5A	0.5A	0.5A	0.5A	-2.0A	0A	0A	0A	0A	0A
First photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	-2.0A	0.5A	0.5A	0.5A	0.5A
Second photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	-2.0A	0.5A	0.5A	0.5A
Third photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	0.5A	-2.0A	0.5A	0.5A
Fourth photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	0.5A	0.5A	-2.0A	0.5A
Fifth photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	0.5A	0.5A	0.5A	-2.0A

Furthermore, an input voltage of the DC-DC circuit is Vpv, a sum of the currents of the normal photovoltaic strings is Isum, and a resistor connected between the switch and negative electrode has resistance Rg. An insulation impedance Riso- of the negative electrode to the ground is calculated according to Ohm's law as: Riso-=Vpv/Isum-Rg.
The current detection devices in the above embodiments are provided at the branches where the photovoltaic strings are located. An implementation in which current detection devices are provided at junction branches is described hereinafter in conjunction with the drawings.
The photovoltaic power generation system according to the embodiment of the present disclosure further includes n-1 current detection devices. Negative electrodes of the n photovoltaic strings are connected to each other to form n-1 negative junctions. n-2 current detection devices of the n-1 current detection devices each are connected between two adjacent negative junctions of the n-1 negative junctions, and the remaining one current detection device (such as, the current detection device W11 in Figure 6) of the n-1 current detection devices is connected between a negative electrode of a first photovoltaic string and a negative junction corresponding to a second photovoltaic string.
Reference is made to Figure 6, which is a schematic diagram of a photovoltaic power generation system in common negative connection according to another embodiment of the present disclosure.
It can be seen from Figure 4 and Figure 6 that the location of the current detection device shown in Figure 4 is different from the location of the current detection device shown in Figure 6. A current detection device W12 is taken as an example. The current detection device W12 in Figure 4 detects a current of a branch where a second photovoltaic string connected to the first DC-DC circuit is located, while the current detection device W12 in Figure 6 detects a current that is a sum of currents of a first photovoltaic string and a second photovoltaic string that are connected to the first DC-DC circuit. Currents detected by other current detection devices are determined in a similar way, which are not described in detail herein.
As shown in Figure 6, the photovoltaic power generation system is in common negative connection, that is, negative input terminals and negative output terminals of all the DC-DC circuits are connected to each other.
As shown in Figure 6, in the photovoltaic power generation system in common negative connection, negative input terminals and negative output terminals of all the DC-DC circuits are connected to each other. Assuming that an insulation fault occurs at a negative electrode of a k-th (0<k<n) photovoltaic string connected to an m-th DC-DC circuit, a current of each of the photovoltaic strings is Imn, and a direction of current flowing from a current detection device Wmn to the negative electrode of the photovoltaic string is defined as a positive direction. The current flowing through the current detection device Wmn is calculated as: $\begin{matrix} Wmi = (Im 1 + \dots + Imi) & i < k \end{matrix}$ $\begin{matrix} Wmi = - (Im (i + 1) + \dots + Imn + \sum I_{normal}) & i ⩾ k \end{matrix}$
Where, Σ I_normal represents a sum of currents of photovoltaic strings connected to all other normal DC-DC circuits.

For example, two DC-DC circuits are provided, and each of the two DC-DC circuits is connected to five photovoltaic strings. If an insulation fault to the ground occurs at a negative electrode of a first photovoltaic string, a second photovoltaic string, a third photovoltaic string, a fourth photovoltaic string, or a fifth photovoltaic string connected to one of a first DC-DC circuit and a second DC-DC circuit, currents detected by the current detection devices for various photovoltaic strings are shown in Table 3.

Table 3. Currents of photovoltaic strings if a short circuit occurs between a negative electrode of a photovoltaic string and the ground in a photovoltaic power generation system in common negative connection

	W11	W12	W13	W14	W15	W21	W22	W23	W24	W25
First photovoltaic string connected to first DC-DC	-4.5A	-4.0A	-3.5A	-3.0A	-2.5A	0.5A	1.0A	1.5A	2.0A	2.5A
Second photovoltaic string connected to first DC-DC	0.5A	-4.0A	-3.5A	-3.0A	-2.5A	0.5A	1.0A	1.5A	2.0A	2.5A
Third photovoltaic string connected to first DC-DC	0.5A	1.0A	-3.5A	-3.0A	-2.5A	0.5A	1.0A	1.5A	2.0A	2.5A
Fourth photovoltaic string connected to first DC-DC	0.5A	1.0A	1.5A	-3.0A	-2.5A	0.5A	1.0A	1.5A	2.0A	2.5A
Fifth photovoltaic string connected to first DC-DC	0.5A	1.0A	1.5A	2.0A	-2.5A	0.5A	1.0A	1.5A	2.0A	2.5A
First photovoltaic string connected to second DC-DC	0.5A	1.0A	1.5A	2.0A	2.5A	-4.5A	-4.0A	-3.5A	-3.0A	-2.5A
Second photovoltaic string connected to second DC-DC	0.5A	1.0A	1.5A	2.0A	2.5A	0.5A	-4.0A	-3.5A	-3.0A	-2.5A
Third photovoltaic string connected to second DC-DC	0.5A	1.0A	1.5A	2.0A	2.5A	0.5A	1.0A	-3.5A	-3.0A	-2.5A
Fourth photovoltaic string connected to second DC-DC	0.5A	1.0A	1.5A	2.0A	2.5A	0.5A	1.0A	1.5A	-3.0A	-2.5A
Fifth photovoltaic string connected to second DC-DC	0.5A	1.0A	1.5A	2.0A	2.5A	0.5A	1.0A	1.5A	2.0A	-2.5A

It can be seen from the last row of Table 3 that, a current detected by a current detection device for a photovoltaic string where the insulation fault to the ground occurs is negative and has a maximum magnitude. For example, if the insulation fault to the ground occurs at the negative electrode of the fifth photovoltaic string connected to the second DC-DC circuit, a current detected by the current detection device W25 is -2.5A, that is, the current is negative and has a maximum magnitude in the last row.
The photovoltaic power generation system shown in Figure 6 is in common negative connection. Implementation in which a photovoltaic power generation system not in common negative connection is described hereinafter.
Reference is made to Figure 7, which is a schematic diagram of a photovoltaic power generation system not in common negative connection according to another embodiment of the present disclosure.
It can be seen from Figure 5 and Figure 7 that the location of the current detection device shown in Figure 5 is different from the location of the current detection device shown in Figure 7. A current detection device W12 is taken as an example. The current detection device W12 in Figure 5 detects a current of a branch where a second photovoltaic string connected to the first DC-DC circuit is located, while the current detection device W12 in Figure 7 detects a current that is a sum of currents of a first photovoltaic string and a second photovoltaic string that are connected to the first DC-DC circuit. Currents detected by other current detection devices are determined in a similar way, which are not described in detail herein.
The photovoltaic power generation system shown in Figure 7 is not in common negative connection, that is, a negative input terminal and a negative output terminal of each of the DC-DC circuits are disconnected from each other.
Assuming that an insulation fault occurs at a negative electrode of a k-th (0<k<n) photovoltaic string connected to an m-th DC-DC circuit, a current of each of the photovoltaic strings is Imn, and a direction of current flowing from a current detection device Wmn to the positive electrode of the photovoltaic string is defined as a positive direction. The current flowing through the current detection device Wmn is calculated as: $\begin{matrix} Wmi = (Im 1 + \dots + Imi) & i < k \end{matrix}$ $\begin{matrix} Wmi = - (Im (i + 1) + \dots + Imn) & i ⩾ k \end{matrix}$

For example, two DC-DC circuits are provided, and each of the two DC-DC circuits is connected to five photovoltaic strings. If an insulation fault to the ground occurs at a negative electrode of a first photovoltaic string, a second photovoltaic string, a third photovoltaic string, a fourth photovoltaic string, or a fifth photovoltaic string connected to one of a first DC-DC circuit and a second DC-DC circuit, currents detected by the current detection devices for various photovoltaic strings are shown in Table 4.

Table 4. Currents of photovoltaic strings if a short circuit occurs between a negative electrode of a photovoltaic string and the ground in a photovoltaic power generation system not in common negative connection

	Wil	W12	W13	W14	W15	W21	W22	W23	W24	W25
First photovoltaic string connected to first DC-DC	-2.0A	-1.5A	-1.0A	-0.5A	0A	0A	0A	0A	0A	0A
Second photovoltaic string connected to first DC-DC	0.5A	-1.5A	-1.0A	-0.5A	0A	0A	0A	0A	0A	0A
Third photovoltaic string connected to first DC-DC	0.5A	1.0A	-1.0A	-0.5A	0A	0A	0A	0A	0A	0A
Fourth photovoltaic string connected to first DC-DC	0.5A	1.0A	1.5A	-0.5A	0A	0A	0A	0A	0A	0A
Fifth photovoltaic string connected to first DC-DC	0.5A	1.0A	1.5A	2.0A	0A	0A	0A	0A	0A	0A
First photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	-2.0A	-1.5A	-1.0A	-0.5A	0A
Second photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	-1.5A	-1.0A	-0.5A	0A
Third photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	1.0A	-1.0A	-0.5A	0A
Fourth photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	1.0A	1.5A	-0.5A	0A
Fifth photovoltaic string connected to second DC-DC	0A	0A	0A	0A	0A	0.5A	1.0A	1.5A	2.0A	0A

It can be seen from the first row of Table 4 that, a current detected by a current detection device for a photovoltaic string where the insulation fault to the ground occurs is negative and has a maximum magnitude. For example, if the insulation fault to the ground occurs at the negative electrode of the first photovoltaic string connected to the first DC-DC circuit, a current detected by the current detection device W11 is -2.0A, that is, the current is negative and has a maximum magnitude in the first row.
In the photovoltaic power generation system according to the embodiments of the present disclosure, in addition to the diode, the switch is connected to a current limiting device in series. The type of the current limiting device is not limited herein. For example, the current limiting device may be a current limiting resistor. Detailed descriptions are provided hereinafter in conjunction with the drawings.
A location of the current limiting device is not limited in the embodiment of the present disclosure, as long as the current limiting device and the switch are connected in series to form a branch, and the branch is connected between the positive input terminal of the corresponding DC-DC circuit and the ground. Hereinafter, for example, the current limiting device is a current limiting resistor.
Reference is made to Figure 8, which is a schematic diagram illustrating a photovoltaic power generation system including a current limiting resistor according to an embodiment of the present disclosure.
The current limiting resistor R shown in Figure 8 is connected between the switch K and cathodes of diodes. It should be understood that, the number of the diodes is equal to the number of the DC-DC circuits. A positive input terminal of each of the DC-DC circuits is connected to an anode of the corresponding diode. M DC-DC circuits and the diodes D1 to DM are in one-to-one correspondence. The cathodes of the diodes are connected to the switch K through the current limiting resistor R.
The current limiting resistor shown in Figure 8 is provided at a main branch, and thus only one current limiting resistor is provided. That is, only one current limiting resistor and one switch are provided.
Reference is made to Figure 9, which is a schematic diagram illustrating a photovoltaic power generation system including current limiting resistors according to another embodiment of the present disclosure.
As shown in Figure 9, for each of the DC-DC circuits, the current limiting resistor R corresponding to the DC-DC circuit is connected between an anode of the diode corresponding to the DC-DC circuit and a positive input terminal of the DC-DC circuit. It should be understood that, the number of the diodes is equal to the number of the DC-DC circuits. M DC-DC circuits and the diodes D 1 to DM are in one-to-one correspondence, and the anode of each of the diodes is connected to the positive input terminal of the corresponding DC-DC circuit through the corresponding current limiting resistor R.
As shown in Figure 9, each current limiting resistor is connected at a branch, and each of the DC-DC circuits is connected to one current limiting resistor. That is, the number of the current limiting resistors is equal to the number of the DC-DC circuits.
In the photovoltaic power generation system according to the embodiments of the present disclosure, if an insulation fault occurs at the negative electrode of the photovoltaic power generation system to the ground, the insulation fault can be located at a specific photovoltaic string, which is suitable for a scenario that multiple DC-DC circuits are connected to one DC-AC circuit in the photovoltaic power generation system. Furthermore, current detection devices in the photovoltaic power generation system may be directly used, only the diode and one switch are additionally provided, so that hardware is easy to implement and has low cost. Moreover, the impedance to ground is calculated based on a photovoltaic voltage and currents of the photovoltaic strings already detected in the photovoltaic power generation system, further improving the detection precision of the insulation impedance.
Based on the photovoltaic power generation system according to the above embodiments, an insulation detection method is further provided according to an embodiment of the present disclosure, which is described in detail hereinafter in conjunction with the drawings.
Reference is made to Figure 10, which is a flowchart of an insulation detection method for a photovoltaic power generation system according to an embodiment of the present disclosure.
With the insulation detection method for a photovoltaic power generation system according to the embodiment of the present disclosure, the photovoltaic power generation system includes a switch, a DC-AC circuit and at least one DC-DC circuit. An input end of each of the at least one DC-DC circuit is configured to connect n photovoltaic strings, n is an integer greater than or equal to two, and an output end of the at least one DC-DC circuit is connected to an input end of the DC-AC circuit. Positive electrodes of all the photovoltaic strings are grounded through the switch. The method includes the following steps S1001 and S1002.
In step S1001, it is determined whether an insulation fault occurs at a negative electrode of the photovoltaic power generation system to ground based on an insulation impedance of the photovoltaic power generation system to the ground.
In step S1002, if the insulation fault occurs at the negative electrode of the photovoltaic power generation system to the ground, the switch is closed and a photovoltaic string where the insulation fault occurs is determined based on a magnitude and a direction of a current of each of the photovoltaic strings.
A solution for determining a photovoltaic string where the insulation fault to the ground occurs is provided according to the embodiment of the present disclosure as follows. If it is detected that an insulation fault occurs at a negative electrode of the photovoltaic power generation system to the ground, the photovoltaic power generation system is controlled to stop operating, positive electrodes of all the photovoltaic strings are controlled to be grounded, and the photovoltaic string where the insulation fault to the ground occurs is directly determined based on a magnitude and a direction of a current of each of the photovoltaic strings.
It should be understood that, if an insulation fault occurs at the photovoltaic power generation system to the ground, it is determined whether the insulation fault occurs based on an insulation impedance of the photovoltaic power generation system to the ground. For example, if the insulation impedance of the photovoltaic power generation system to the ground is less than a preset impedance, it indicates that the insulation fault occurs. According to the embodiment of the present disclosure, if the insulation fault occurs at the negative electrode of the photovoltaic power generation system, the insulation fault is further accurately located.
If the insulation fault occurs at the negative electrode of the photovoltaic power generation system, the positive electrodes of all the photovoltaic strings are controlled to be grounded, and a short circuit occurs between a negative electrode of a photovoltaic string where the insulation fault occurs and the ground. As a result, a current path is formed between the negative electrode of the photovoltaic string and the ground, and currents of other photovoltaic strings flow into the photovoltaic string where the insulation fault occurs, in a direction opposite to a direction of a current of the photovoltaic string where the insulation fault occurs. Therefore, the photovoltaic string where the insulation fault occurs can be determined by detecting the magnitude and the direction of the current of each of the photovoltaic strings according to the embodiments of the present disclosure.
In an embodiment, the photovoltaic power generation system further includes at least n-1 current detection devices. Negative electrodes of the n photovoltaic strings are connected to each other. Each of the at least n-1 current detection devices is connected between a negative electrode of a corresponding photovoltaic string and a negative junction corresponding to the photovoltaic string, and the current detection device is configured to detect a current of a branch where the photovoltaic string is located.
The photovoltaic string where the insulation fault occurs is determined based on the magnitude and the direction of the current of each of the photovoltaic strings by: determining, if a current of an i-th photovoltaic string is detected as negative, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, where the i-th photovoltaic string is one of the n photovoltaic strings.
In an embodiment, the photovoltaic power generation system further includes n-1 current detection devices. Negative electrodes of the n photovoltaic strings are connected to each other to form n-1 negative junctions, n-2 current detection devices of the n-1 current detection devices each are connected between two adjacent negative junctions of the n-1 negative junctions, and the remaining one current detection device of the n-1 current detection devices is connected between a negative electrode of a first photovoltaic string and a negative junction corresponding to a second photovoltaic string.
The photovoltaic string where the insulation fault occurs is determined based on the magnitude and the direction of the current of each of the photovoltaic strings by: determining, if a current of an i-th photovoltaic string is detected as negative and an absolute value of the current is maximum, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground.
With the insulation detection method for a photovoltaic power generation system according to the embodiments of the present disclosure, if an insulation fault occurs at a negative electrode of the photovoltaic power generation system to the ground, the insulation fault can be located at a specific photovoltaic string, which is suitable for a scenario that multiple DC-DC circuits are connected to one DC-AC circuit in the photovoltaic power generation system. Furthermore, current detection devices in the photovoltaic power generation system may be directly used, only the diode and one switch are additionally provided, so that hardware is easy to implement and has low cost. Moreover, the impedance to ground is calculated based on a photovoltaic voltage and currents of the photovoltaic strings already detected in the photovoltaic power generation system, further improving the detection precision of the insulation impedance.
It should be noted that, various embodiments in this specification are described in a progressive manner. Each of the embodiments focuses on its differences from other embodiments, and references may be made of these embodiments with respect to the same or similar parts. Since the system or device disclosed in the embodiments corresponds to the method disclosed in the embodiments, the description of the system or device is relatively simple, and reference may be made to the description of the method for relevant parts.
Based on the above description of the disclosed embodiments, those skilled in the art can carry out or use the present disclosure. It is apparent for those skilled in the art to make many modifications to these embodiments. The general principle defined herein may be applied to other embodiments without departing from scope of the present disclosure. Therefore, the present disclosure is not limited to the embodiments illustrated herein, but should be defined by the broadest scope consistent with the principle and novel features disclosed herein.

Claims

A photovoltaic power generation system, comprising
a switch;

a direct current-alternating current, DC-AC, circuit;

a controller and

at least one DC-DC circuit, wherein

an input end of each of the at least one DC-DC circuit is configured to connect n photovoltaic strings, wherein n is an integer greater than or equal to two;

an output end of the at least one DC-DC circuit is connected to an input end of the DC-AC circuit;

positive electrodes of all the photovoltaic strings are grounded through the switch; and the controller is configured to close the switch and determine a photovoltaic string where an insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings, if the insulation fault occurs at a negative electrode of the photovoltaic power generation system to ground.
The photovoltaic power generation system according to claim 1, further comprising at least n-1 current detection devices, wherein
negative electrodes of the n photovoltaic strings are connected to each other; and

each of the at least n-1 current detection devices is connected between a negative electrode of a corresponding photovoltaic string and a negative junction corresponding to the photovoltaic string, and the current detection device is configured to detect a current of a branch where the photovoltaic string is located.
The photovoltaic power generation system according to claim 2, wherein the controller is configured to determine, if a current of an i-th photovoltaic string is detected as negative, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, wherein the i-th photovoltaic string is one of the n photovoltaic strings.
The photovoltaic power generation system according to claim 1, further comprising n-1 current detection devices, wherein
negative electrodes of the n photovoltaic strings are connected to each other to form n-1 negative junctions; and

n-2 current detection devices of the n-1 current detection devices each are connected between two adjacent negative junctions of the n-1 negative junctions, and the remaining one current detection device of the n-1 current detection devices is connected between a negative electrode of a first photovoltaic string and a negative junction corresponding to a second photovoltaic string.
The photovoltaic power generation system according to claim 4, wherein the controller is configured to determine, if a current of an i-th photovoltaic string is detected as negative and an absolute value of the current is maximum, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, wherein the i-th photovoltaic string is one of the n photovoltaic strings.
The photovoltaic power generation system according to claim 3 or 5, wherein a negative input terminal and a negative output terminal of the at least one DC-DC circuit are connected to each other.
The photovoltaic power generation system according to claim 3 or 5, wherein a negative input terminal and a negative output terminal of the at least one DC-DC circuit are disconnected from each other.
The photovoltaic power generation system according to any one of claims 1 to 7, further comprising a diode, wherein
for each of the at least one DC-DC circuit,

positive electrodes of the n photovoltaic strings connected to the input end of the DC-DC circuit are connected to an anode of the diode corresponding to the DC-DC circuit, a cathode of the diode is connected to a first terminal of the switch, and a second terminal of the switch is grounded.
The photovoltaic power generation system according to claim 8, further comprising a current limiting device, wherein
the number of the current limiting device and the number of the at least one DC-DC circuit are equal to each other; and

for each of the at least one DC-DC circuit,

the current limiting device corresponding to the DC-DC circuit, the diode corresponding to the DC-DC circuit and the switch are connected in series to form a branch, and the branch is connected between a positive input terminal of the DC-DC circuit and the ground.
The photovoltaic power generation system according to claim 8, further comprising a current limiting device, wherein
the number of the current limiting device and the number of the switch each are one; and for each of the at least one DC-DC circuit,

the current limiting device, the diode corresponding to the DC-DC circuit and the switch are connected in series to form a branch, and the branch is connected between a positive input terminal of the DC-DC circuit and the ground.
An insulation detection method for a photovoltaic power generation system, wherein the photovoltaic power generation system comprises a switch, a direct current-alternating current, DC-AC, circuit and at least one DC-DC circuit, wherein an input end of each of the at least one DC-DC circuit is configured to connect n photovoltaic strings, n is an integer greater than or equal to two; an output end of the at least one DC-DC circuit is connected to an input end of the DC-AC circuit; and positive electrodes of all the photovoltaic strings are grounded through the switch,
wherein the insulation detection method comprises:
closing the switch and determining a photovoltaic string where the insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings, if the insulation fault occurs at the negative electrode of the photovoltaic power generation system to the ground.
The insulation detection method according to claim 11, wherein
the photovoltaic power generation system further comprises at least n-1 current detection devices, negative electrodes of the n photovoltaic strings are connected to each other, each of the at least n-1 current detection devices is connected between a negative electrode of a corresponding photovoltaic string and a negative junction corresponding to the photovoltaic string, and the current detection device is configured to detect a current of a branch where the photovoltaic string is located,

wherein, the determining a photovoltaic string where the insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings comprises:
determining, if a current of an i-th photovoltaic string is detected as negative, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground, wherein the i-th photovoltaic string is one of the n photovoltaic strings.
The insulation detection method according to claim 11, wherein
the photovoltaic power generation system further comprises n-1 current detection devices, negative electrodes of the n photovoltaic strings are connected to each other to form n-1 negative junctions; n-2 current detection devices of the n-1 current detection devices each are connected between two adjacent negative junctions of the n-1 negative junctions, and the remaining one current detection device of the n-1 current detection devices is connected between a negative electrode of a first photovoltaic string and a negative junction corresponding to a second photovoltaic string; and

wherein, the determining a photovoltaic string where the insulation fault occurs based on a magnitude and a direction of a current of each of the n photovoltaic strings comprises:
determining, if a current of an i-th photovoltaic string is detected as negative and an absolute value of the current is maximum, that the insulation fault occurs at a negative electrode of the i-th photovoltaic string to the ground.